Such methods are taught, for example, by West German Utility Model No. 8,212,094. In the prior art process, the pressure in the gas compartment at which the gas displaces the liquid from the largest pore of the membrane or filter material is determined by measuring the differential pressure between the liquid compartment and the surroundings, the change of this differential pressure over time being used as a measure for the rate of flow of the gas through the pores. For relatively large pores, the flow rate can also be measured directly (ASTM F 316-70), or the rising of bubbles can be observed visually (ASTM E 128-61).
However, as explained in West German Pat. No. 3,306,647, in these prior art methods it has been observed that with an increase in size of the membrane area to be examined and of the pore diameters present in the membrane, a certain gas flow rate can be noted even before the pressure determined by the bubble point has been reached, so that the measured result with respect to the bubble point and the largest pore is distorted. For example, this flow rate can be caused by a diffusion flow, which is causally related to the solubility--proportional to the partial pressure--of gases in liquids and the resulting concentration gradients of the gas molecules in the pores of the membrane. Although, according to the method described in West German Pat. No. 3,306,647, this undesired phenomenon is prevented, the use of the method described therein is very time-consuming, so that only with a large staff is it possible to examine the number of membranes or of membrane modules necessary for production control.